My first piece of advice is to ignore the concept of magnification (e.g. 4x, 7x etc...) - it is really a meaningless term.

To specifically answer your question about which lense will allow you to 'fill the frame' with a distant object - the answer is the lense with the longer maximum focal range. So you have one lense with a max focal length of 200mm and one with 300mm. The one with 300mm will allow objects farther away to 'fill the frame' more.

But as you can guess there are other factors. First and foremost is that the 70-300 will not be very useful for objects that are close to you or wide scenery - so, if you want to take a picture of something within 10 feet or so you MAY have to change lenses.

i am in agreement.. check out the sigma 70-200 2.8 ex dg hsm.. its simply a great value.. it offers L-series sharpness and contrast and AF speed (hsm=usm) for a fraction of the price... no brainer.. also, if you haven't why don't you check out the sigma 100-300 F4.0 ex hsm.. simply might be the best lens sigma makes right now.. and its only 100USD more than the 70-200 and not much bigger..
just a few more options to look into..

I agree with the agreement... I would have gotten a better lens had i had your budget. You're going to be married to your lens so make sure you're love.

I seem to be still Wandering around for perfect lens for my Rebel XT . By going through many forums and having hands on experience on few models of lens I have decided to increase my budget to around 600£ and buy a decent lens.

Could you guys tell me if

1)Sigma 70-200 F/2.8 Ex HSM is a good choice ? though the 200mm is not really enough for me I feel adding a 2X converter would serve my needs. at the moment canon 70-200 F2.8 USM is still out of reach for me.

2) or just try with some cheap lens like sigma 70-300 F 4/5.6 DL macro super for some time as I am interested in macro photography too.(till some new model is out or the prices drop). I am not expecting professional quality pics but atleast semi pro.

I seem to be still Wandering around for perfect lens for my Rebel XT . By going through many forums and having hands on experience on few models of lens I have decided to increase my budget to around 600£ and buy a decent lens. Could you guys tell me if1)Sigma 70-200 F/2.8 Ex HSM is a good choice ? though the 200mm is not really enough for me I feel adding a 2X converter would serve my needs. at the moment canon 70-200 F2.8 USM is still out of reach for me.2) or just try with some cheap lens like sigma 70-300 F 4/5.6 DL macro super for some time as I am interested in macro photography too.Pradeep

First of all, the "x" factor is nothing more than the ratio of the longest focal length to the shortest for that particular zoom.

You mention a need for macro. Most lenses labeled "macro" should be called"semi macro", as MACRO implies an image with a 1:1 ratio to the actual subject. 1:2 is not macro. What are you going to shoot in macro mode?

The Sigma 70-300 APO Macro Super II is a pretty good semimacro. The DL macro is not. There have been some extraordinary photos shown on various forums with the 70-300 APO. I suspect many nature and wildlife photographers have bought that lens as their first telephoto zoom. Then they moved to 400mm. Then they moved to 500mm. The fact is you never have a long enough lens for birds. While I am happy enough with the Canon 100-400LIS on my XT, my wife can only shoot with her 500mm F4LIS, usuallywith a Canon 1.4xII extender and sometimes with a2xII extenderon the 20D. It is an expensive combination. The extender alone costs half again as much as the Sigma 70-300 APO Macro Super II--which happens to be the very telephoto zoom she started with!

Here's something I believe about telephoto lenses: Don't buy a lens because "it will be long enough with a 1.4x or 2x converter". That rarely works out. You should get the lens with the focal length you think you need. A TC reduces image quality. A cheap TC reduces image quality a lot. A 2x degrades the image more than a 1.4x. A cheap 2x converter and, well, you get the idea. Put a 2x on that Sigma 70-200 F2.8 and you have a not-so-great 400mm F5.6 You will be better off saving for a 400mm. Canon has a 400mmL and a 100-400mmLIS. Sigma has an excellent 80-400mm with OS (Sigma's version of IS). Sigma also makes a 50-500mm F4-F6.3 zoom that is better than it should be. I have one of those, too. Bought for $782 before it became so popular, it is a pretty sharp, very heavy, zoom lens. Incidentally, Sigma's top of the line lenses are all designated EX.

Image Stabilization. Canon is the industry leader in IS lenses with more lenses with image stabilization than the others combined. After using both, especially at longer focal lengths, it is very difficult to go back. IS lenses cost more. L lenses cost more. LIS lenses cost too much! The first responder to your message, JohnG, asked good questions and gave good advice. Assuming you take care of it, a good lens may be with you for a long time after your XT is in the back of a closet with the Pachinko machine. A cheap lens may end up in that closet with the XT. On the other hand, a good quality low priced lens may be what helps you to decide what kind of photography you really like. My wife would not have considered the Canon 500mm F4LIS at the time she bought the Sigma 70-300APO. The Sigma gave her a start into nature photography, though. So for that alone, the $165 she spent on that lens (it has become quite popular since then, and the price has gone up) was money well spent.

One more thing. Be sure you get objective reviews and opinions as basis for your purchase decisions and don't get caught up in anonymous forum ravings. To his credit, JohnG did not suggest any specific lens to you. Be careful of people suggesting "their" lens as what you need. They may be subconsciously looking for vindication of their own purchase decisions.

you would be better off with the 100-300 with a 1.4 teleconverter when you need it than you will be using a 2x teleconverter on the 70-200.. and its only 60USD more for the 100-300 f4.0 vs the 70-200 f2.8..

Thanks a lot for the quick reply especially William (wburychka), that was a good lenthy advice. I appreciate that.

I think you are right instead of buying a teleconverter as an addon, its better to buy a single lens. But the problem is to find out the focal lenght of your choice for regular use. As you suggested buying a cheap but yet decent lens like sigma 70-300 APO macro and trying to find out the exact needs is a good idea. I had this idea too but on reading many msg's advicing to buy a good lens on the first instance than trying with some low end lenses confused me, as I am brand new to Digital SLR or for that matter SLR cameras. My rebel XT is the first Digital camera I have used ever. But to jump ahead and buy a expensive lens for guys like me is not a joke. Either I have to depend on guys like you for your valuable advice or just gamble. but each person has his own interests and his own range of focal lengths. As William mentioned people tend to promote lenses which they own. but to certain extent that is the main intention of forums like this, but can be harzadous if you choose a wrong lense based on the forum review particularly if it does not suit your needs.

As I am still in search for my(Perfect ) lens I feel my answers will not be answered till I start usingsome lens. So I have decided to buy the sigma 70-300 Apo F4/5.6 Macro super and find out my exact needs as somtimes things are different as what you think and what you end up doing.

Thanks for all the feedback. But have another doubt. There are many varities of Sigma 70-300 lens. which one is best for my Rebel XT?

I have to say this about a lens from what I have noticed.Alot of taking good pics is about you.I have noticed a sttep learning curve going from a point and shoot to a DSLR.I have the 20D and due to money constraints I bought the Sigma 70-300mm APO F4-5.6 Super II Macro for the 300mm length and for the macro.I like the lens alot but at 300mm shake like a 100 yr old man.I have taken some pics that I feel are really nice.I am still learning and saving for a IS lens.I would recommend this lens if you are on a budget.

style="BACKGROUND-COLOR: #000000"

style="BACKGROUND-COLOR: #000000"Here are a few of my better pics.Dont know if they are great but I like them.